The contamination of surface and near-surface soils has become a matter of great concern in many locations. Soil may become contaminated with chemical, biological, and/or radioactive contaminants. Material spills, leaking storage vessels, and landfill seepage of improperly disposed materials are just a few examples of the many ways in which soil may become contaminated. If left in place, many of these contaminants will find their way into aquifers, air, or into the food supply, and could become public health hazards.
There are many proposed methods for removal of surface contaminants, such as excavation followed by incineration, in situ vitrification, biological treatment, chemical additives for deactivation, radiofrequency heating, etc. Although successful in some applications, these methods can be very expensive and are not practical if many tons of soil must be treated.
A process that may be used to remove contaminants from subsurface soil is a soil vapor extraction process. In such process a vacuum is applied to the soil to draw air and vapor through subsurface soil. The vacuum may be applied at a soil/air interface, or the vacuum may be applied through vacuum wells placed within the soil. The air and vapor may entrain and carry volatile contaminants towards the source of the vacuum. Off-gas removed from the soil by the vacuum which includes contaminants that were within the soil is then transported to a treatment facility wherein it is processed to eliminate, or reduce contaminants to acceptable levels. The disadvantage of this method is that the method is limited to the extraction of contaminants present in vapor form.
In situ thermal desorption may be used to increase the effectiveness of a soil vapor extraction process, the vaporization of soil contaminants may be supported by thermal desorption. In situ thermal desorption involves in situ heating of the soil to raise the temperature of the soil while simultaneously removing off-gas from the soil. Heat added to contaminated soil may raise the temperature of the soil above vaporization temperatures of contaminants within the soil and cause the contaminants to vaporize. A vacuum applied to the soil allows drawing of the vaporized contaminant out of the soil.
A method of heating a soil containing contaminants comprising the injection of a heated fluid into the soil is, for instance described in EP 1604749. The herein described method consists of introducing a system of perforated columns into the soil. A stream of hot air is sent through the columns. The hot air is injected into the soil through perforations in the columns at the level of the pipe perforations. A contaminant vapor is formed in the soil, which may be removed from the soil through the perforations in the columns and disposed to an off-gas treatment unit.
A major drawback of this method is that large amounts of energy are required for the heating process, much of which is lost upon transport to or from the soil. The vaporized contaminants travel long distances through heat-conductive, non-isolated material before a treatment facility is reached. Fuel required for the heating process is expensive. From an energy point of view, the method is costly and not environmentally friendly. Moreover, the perforated columns are expensive. The major part of the cost is due to the perforation of the columns. Although the columns may be re-used a limited number of times in the same application, their recycling is limited to the same type of soil and/or contamination. The storage of these columns prior to their re-employability is expensive.
Hence, there remains a need in the art to reduce the cost of the process further and to reduce its carbon foot-print. The present invention aims to provide a solution to at least one of the above-mentioned problems by providing a methods and devices for cleaning a soil containing contaminants which provides energy savings, is less expensive, easy to install, easy to use and is suitable for use in follow-on applications. In particular, the present invention aims to provide methods and systems for soil remediation wherein heat-losses are reduced as transport of heated fluids is kept to a minimum. Moreover, the present invention aims to provide a method and system for soil remediation wherein the perforated columns that have been driven into the soil for cleaning need not be removed after the soil has been remediated but can be re-used for other purposes.